Base stations used in wireless telecommunication systems have the capability to receive linear polarized electromagnetic signals. These signals are then processed by a receiver at the base station and fed into the telephone network. In practice, the same antenna which receives the signals can also be used to transmit signals if the transmitted signals are at different frequencies than the received signals.
Wireless telecommunication systems suffer from the problem of multi-path fading. Diversity reception is often used to overcome the problem of severe multi-path fading. A diversity technique requires at least two signal paths that carry the same information but have uncorrelated multi-path fadings. Several types of diversity reception are used at base stations in the telecommunications industry including space diversity, direction diversity, polarization diversity, frequency diversity, and time diversity. A space diversity system receives signals from different points in space requiring two antennas separated by a significant distance. Polarization diversity uses orthogonal polarization to provide uncorrelated paths.
As is well-known in the art, the sense or direction of polarization of an antenna is measured from a fixed axis and can vary, depending upon system requirements. In particular, the sense of polarization can range from vertical polarization (0 degrees) to horizontal polarization (90 degrees). Currently, the most prevalent types of polarization used in systems are those which use vertical/horizontal and +45.degree./-45.degree. polarization ("slant 45.degree."). However, other angles of polarization can be used. If an antenna receives or transmits signals of two polarizations normally orthogonal, they are also known as dual polarized antennas.
Dual polarized antennas have to meet a certain port-to-port coupling or isolation specification. The typical port-to-port isolation specification is -30 dB. Furthermore, many dual polarized antennas are designed with microstrip lines integrated with aperture coupled radiating patches due to the associated lower manufacturing cost and the desirable slim profile. The present invention discloses a means to lower the port-to-port isolation of dual polarized antenna systems with some simple parasitic coupling strips placed on the non-radiative side of the panel antenna.
Generally, dual polarized antennas must meet the -30 dB isolation specification in order to be marketable. Not meeting the specification means the system integrator might have to use higher performance filters which cost more and decrease antenna gain. The present invention overcomes these concerns because it meets the -30 dB isolation specification.
Moreover, the visual impact of base station towers on communities has become a societal concern. It has become desirable to reduce the size of these towers and thereby lessen the visual impact of the towers on the community. The size and scale of the towers can be reduced by using base station towers with fewer antennas. This can be achieved if dual polarized antennas and polarization diversity are used. Such systems replace systems using space diversity which require pairs of vertically polarized antennas. Some studies indicate that, for urban environments, polarization diversity provides an equivalent signal quality to space diversity. With the majority of base station sites located in urban environments, it is likely that dual polarized antennas will be used in place of the conventional pairs of vertically polarized antennas.